Hydraulic filters for filtering for example hydraulic oil often have a full-flow filter and a partial-flow, the full-flow filter having the task of filtering the hydraulic liquid during normal operation, whereas the partial-flow filter assumes a permanent oil maintenance function. Previously, the partial-flow filter was usually arranged in a separate filter housing and supplied with a completely separate partial flow or a small partial flow branched off from the full flow. Consequently, two housings were needed for operation, which resulted in not inconsiderable costs.